Stress-gradient anchor



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Dec. 15, 1964 C. WILUAMS 3,160,988

STRESS-GRADIENT ANCHOR Filed Dec. 16. 1960 2 Sheets-Sheet l INVENToR.

ig. 3. j omey Dec. 15, 1964 c. 1. WILLIAMS STRESS-GRADIENT ANCHOR 2 Sheets-Sheet 2 Filed Deo. 16, 1960 INVENTOR.

United States Patent O 3,163,988 STRESS-GRADIENT ANCHR l Chester I. Williams, 347 Greenbrier SE., Grand Rapids, Mich. Filed Dec. I6, "i960, Ser. No. 76,339 S Claims. (Cl. Sti- 431) This invention relates to the conformation of anchors incorporated in poured concrete. Construction projects of substantial size are usually developed in successive superimposed pours of concrete, with the forms for each pour secured to the set pours below. The forces on the forms are tremendous, as a result of the liquid pressure of the uid concrete. These distributed pressures are resisted by securing the forms to anchors that have been imbedded in the previous pours as they were made. It is not uncommon to'apply a tension of ten tons or more to an anchor, and the manner of transfer of this loading Vto `the surrounding concrete is of critical importance. Such loads will commonly lgenerate stresses in the steel rod of which the anchor is made which will approach the ultimate strength of the rod. To minimize the amount of movement` of the rod material with respect to the surrounding concrete, it isV conventional to form these anchors of rod stock which has been severely cold worked to the point that the ultimate strength of the rod is but slightly above kthe yield point.

The ability of theanchor to withstand stresses is also related to the behavior of the concrete itself. yIt is characteristic of concrete that it is weakest in tension, and it will therefore fail on planes at which the tensile stresses are at a maximum. In the case of embedded anchors,

-these planes are normally disposed at approximately 45 degrees with respectl to the axis 'of principal loading on the anchor.

The eifect of the internal tensile stresses in the anchor create a complicated problem. v The generation of stresses of this magnitude inherently create a corresponding strain (stretchinglaccording to a fixedV ratio characteristie of the particular material. This ratio is commonly referred to as the modulus of elasticity. As the steel anchor becomes subject to heavy loading over its entire length, it necessarily tends to stretch mor( ,han does the surrounding concrete. Herein lies the problem. Where Vthe outer end `of the anchor ispositioneda few inches from the surface of theconcrete, and the anchor extends from there to greater depth, it may be presumed that the grip of the concrete over the entire length of the anchor is required *to resist the forces involved.` The grip of the concrete-on the rst few inches of the an- `chor will therefore usually be inadequate to resist the total loading, since the anchor would pull out lthe surrounding concrete if all the resistance were centered there. This presumes a solid locking of the concrete with respect to the anchor at that point.'v This type of failure 'is common, and results in thedislodgment Aof a conical piece coaxial with and surroundingv the anchor, the breakout takingplace 'on the conical plane of maximum ten- `sile'stress in the concrete.: At the instant of break-out, the.l anchor forces are transferred to the next depth of concrete, bringing into playa stress cone of greater axial length; Ther-inner concretemust then resist the anchor yload without the assistance of the concrete near thev sur- .facerv It is the purpose ofl this invention to utilize the ternative, it `permits a smaller and more` economical anchor to be used to transfer'a given load with safety.

, An important characteristic of the invention isa con:

Y, laissera Patented Dec.. l5, 1964 lce formation of the anchor which results in establishing over the length of the anchor a graduated resistance to movement of the anchor with respect to the surrounding concrete. Near the surface, the anchor is permitted to slip slightly as it stretches, rather than establish a positive locking at all points along the length. Load is therefore gradually picked up over the entire length of the anchor, and points of concentration of stress in the concrete are thereby avoided.

It is conventional to use undulated steel rods as anchors, and the security of a particular portion of the anchor in the concrete is determined by the amplitude of the undulation per unit of length and the abruptness of the undulations. Stated in other Words, a sharp kink in the rod will exhibit a greater locking action than a gradually-curved offset. Applicant has discovered that an anchor can create a very desirable stress gradient over its length, and a correspondingly unconcentrated load transfer to the concrete, if it is provided with a conformation having a series of undulations graduated in amplitude per unit of length of undulation. ThisV can be accomplished in several ways, among these being:

(a.) A constant amplitude of undulations combined with a decreasing period (length) ofundulation.

(b) A constant period of undulation, with progressively increasing amplitude.

(c) A progressively increasing sharpness of bends (curvature) at the individual undulations.

Applicant has also discovered that the concentrations of stress near the surface of the concrete can be climinated by providing a tangential relationship of the anchor with respect to the axis of pull at the-point of connection to the anchor. This tends to be an application of the same principles outlined above, if the anchor is `provided with an arcuate or curved contiguration such that each point along the length of the anchor is progressively disposed at a greater angle with respect to the axis of pull. An arcuate formation at a relatively constant radius, or a parabolic configuration (with the point of increased curvature at the inner end of the anchor) will satisfy this requirement. This conguration may be used alone, or in combination with the graduated undulations referred to above. In the latter case, the continuous curvature is applied to the axis of the undulations;` The several features of the invention will be analyzed in further detail through an analysis of the particular embodiments illustrated in the accompanying l drawings. In the drawings:

FIGURE 1 presents a view in side elevation of an anchor formed in a parabolic curve, the anchor itself being of rod stockfwhich is corrugated by the provision of a rolled thread on its surface. i

FIGURE 2` presents a view in side elevation of a modified form of the invention `illustrating an anchor having a progressively increased amplitude of undulations and a substantially constant undulation period, or lengt vThe axis of the undulations is itself formed in a parabolic curve. Y

FIGURE 3 `presents a modified form of the invention .in Whichjthe plane of theundulations is generally per-V pendicular to the plane of the parabolic curvature.

FIGURE 4 presents a modified form of the invention involving a progressively increased curvature of undulations.

FIGURE 5 presents an varcuate formation of the anchor shown-fin FIGURE 4, in side elevation. t

FIGURE6 illustrates a furthermodification of the inventionyshowing undulations Vof substantially constant amplitude and progressively decreasing period.

FIGURE 7 illustrates a cluster of anchor rods secured to a connector. Y

FIGURE 8 illustrates the attachment of an eye itting for a single anchor rod.

The anchor illustrated in FIGURE 1 is essentially a piece of steel rod indicated at 10 with a threaded coupling portion at the outer end 11 emerging from a recess 12 shown in dotted lines in the concrete. The threaded end 11 establishes the axis of pull on the anchor, the loading normally being applied to the anchor by a so-called she-bolt which secures the forms in place. The surface of the rod may be corrugated, if desired, by an annularor helical pattern indicated at 13. The axis of the rod 10 is formed substantially along the curvature of a parabola, with the maximum curvature existing at the inner end portion identified at 14. It will be noted that the curvature of the rod with respect to the axis of pull progressively increases with distance from the point of connection. Stated another way, the angle of a rod element to the axis of pull near the end 14 is greater than the angle of the axis of a rod element nearer the threaded end 11.

In the modification illustrated in FIGURE 2, the anchor 15 is kinked or undulated in a plane approximately the same as the plane of the parabolic curvature of the neutral axis of the undulations. In this modification of the invention, the period (length) of each undulation is relatively constant over the length of the anchor so that the distance between the peaks shown at 16 is substantially the same as that shown at 17. The amplitude ot the undulations, however, progressively increases over the length of the rod from the outer threaded end to the inner end indicated at 18. The amplitude of the undulations may be considered as the distance from the dotted lines 19 or 20 to an undulation axis disposed exactly between these lines.

The modification shown in FIGURE 3 is similar in general principle to that shown in FIGURE 2, except that the undulations are formed in a plane perpendicular to the plane of the parabola of the undulation axis. Tension is applied to the modiiication shown in FIGURE 3 at the threaded outer end 21, and the amplitude of the undulations of both the FIGURE 2 and FIGURE 3 modifications is the primary variable. In most cases, the amplitude of undulations may increase until the distance between the lines 19 and 20 is approximately one and one-half diameters of the rod of which the anchor xs made. In some instances, it may be desirable to increase this final amplitude, as is the case in FIGURE 3. The graduated amplitude will normally not have to exceed the one and one-half diameter rule in order to adequately secure the rod in the concrete, and it will usually decrease the concentration of stresses in the concrete to adhere to this limitation. The manufacture of the anchors will usually be easier with the arrangement shown in FIGURE 2, since the same bending operation which establishes the parabolic configuration can also be used to form the undulations; while in the FIGURE 3 mod1fication,it will normally require two bending operations. In FIGURE 3, the period of the undulations shown at 22 is substantially the same as that identified at 23. For ease of manufacture, it is preferable that the final end 24 (and also the end 18 shown in FIGURE 2) be left of more or less indeterminate configuration, since the shape of this point is not excessively critical.

It should also be noted that the undulations of FIG- URE 2 and FIGURE 3 are essentially arcuate portions at the lateral extremities of the undulation, connected by relatively straight portions which are tangential to the curves. This arrangement is by no means critical, however, and the showing of FIGURES 4 and 6 will illustrate undulations in which no tangential portions are used, the curves leading directly into each other.

The anchor shown in FIGURES 4 and 5 has the threaded outer end 25, with the undulations 26, 27, 28, and 29. The graduations in these undulations are in the degree of curvature, this degree progressively increasing to the right, as shown in FIGURE 4. These undulations are formed by a decrease in the radii of curvature from that shown at 3f) to that indicated at 31. For convenience of manufacture, the radii of the undulations 28 and 29 are the same. In the particular modification shown in FIG- URE 4, the amplitude of the undulations has not been maintained constant.

FIGURE 5 illustrates the anchor shown in FIGURE 4 on a plane perpendicular to that of FIGURE 4, FIG- URES 4 and 5 being in projection. In side elevation, the anchor 32 is formed in an arc of continuous curvature, the radius of curvature being indicated at 33. This curve is tangential to the axis of the threaded end-25, and it will be noted that the angle of the axis 34 of the undulations is disposed at a progressively increasing angle with respect to the axis 35 of the threaded end 25, over the length of the anchor.

The force applied to the anchor at the threaded cnd 36 of the modification shown in FIGURE 6 is transferred through the rod, and is resisted by the engagement of the undulations 37-41 with the surrounding concrete. These undulations are formed with a progressively increasing curvature, as indicated by the decreasing length of the radii 42-47. The amplitude of the undulations is maintained substantially constant throughout the length of the anchor shown inFIGURE 6, the line 48 down through the center of the vcross-section of the rod moving between the boundaries 49 and 50 which are equally displaced on opposite sides of .the axis of the threaded end 36. The extreme inner end V51 may be left at a random configuration to simplify the bending operation, if desired. The anchor shown in FIGURE 6 may be left with the axis of the undulations coincident with the axis of the threaded end 36, or it may be displaced in an arcuate, parabolic, or some other curved arrangement. Under normal conditions, the curvature of the undulation axis is usually arranged to create a downward deviation, since the anchors are frequently embedded not only near a vertical face, but also near the top of the pour. This deviation permits the anchor to engage a deeper portion of the concrete than would be Ythe case if the undulaton axis were an extension of the axis of the threaded end.

All of the modifications illustrated in the drawings involve what may be termed symmetrical undulations, in the sense that the configuration at either side of the peak is closely similar. It is possible to change this to an arrangement in which such an undulation is replaced by an eccentric configuration resembling a step or zig-zag system. Such a conformation can exhibit the graduations in configuration of one undulation with respect to the succeeding one in the same fashion as has been described above but has a much greater tendency to result in concentrations of stress in the surrounding concrete. The increased sharpness of bends also has the tendency to increase the danger of localized cracks in the rod, particularly if the cold-worked form of r'od is used to obtain the maximum yield point. Y To further illustrate this eccentric type of arrangement, it is possible to provide a generally triangular undulation in Which the waves nearest the threaded end of the'anchor Were in the form of the sides of isosceles triangles, proceeding toward the inner end of the anchor where this arrangement progressively gave way to a shape approaching a right angle. The difficulties in manufacture, together with the likelihood of stress concentrations, make such an arrangement much less to be desired than those shown in the drawings. It may also be noted that the more smoothness of curvature that can be provided with these anchors, the greater is the likelihood of an adequate flow of the poured concrete around them to minimize the possibility of voids at the points of curvature in the undulations. i

FIGURE 7 illustrates an arrangement that is commonly used with other types of anchors where the forces to be 'transmitted at a given degree of set of the concrete ex- 5 ceed the holding power of a single rod. This form of cluster anchor is adaptable to the type of rods discussed herein, and the undulated rods 52-54 are shown welded to the several turns of the helical coil S5 of steel rod which forms a thread `system for engagement with the threaded end 56 of the she-bolt 57.

FIGURE 8 shows an alternative form for the embedded end of an anchor. Prior to and during the pouring of the concrete, the positioned anchors present troublesome points on which to accidentally catch tools, equipment, and the clothing of Workmen. The possibility oi physical injury at these points is also present. To minimize these dangers, the end of the anchor 58 may be formed in an eye 59 to present a rounded contour.

TheA particular embodiments of the present invention which have been illustrated and discussed herein are for illustrative purposes only and are not to be considered as a limitation upon the scope` of the appended claims. In these claims, it is my intent to claim the entire invention disclosed herein, except as I am'limited by the prior art.

I claim: Y

1. An achor for engagement with poured concrete, said anchor comprising: at least one rod member having'a threaded coupling portion and an engaging portion extending from said coupling portion, said engaging portion having undulations of increasing curvature proceeding from said coupling portion, said undulations being formed With respect to an axis having continuous curval ture and extending in tangential relationship to the axis of said coupling portion.

2. An anchor for engagement with poured concrete, said anchor comprising: at least one rod member having a threaded coupling portion and an engaging portion extending from said coupling portion, said engaging portion having undulations of increasing ratio of amplitude to period proceeding fromvsaid coupling portion, said undulations being formed with respect to an axis having continuous curvature andv extending in tangential relationship to the Vaxis of said coupling portion.

3. An anchor for engagement with poured concrete, said anchor comprising: at least one rod member having a threaded coupling portion and anengaging portion extending fromr saidcoupling portion, said engaging portion having undulations of increasing curvature proceeding from said coupling portion, said undulations being formed with respect to an axis extending in tangential relationship to `the axis of said coupling portion.

4. An anchor for engagement with poured concrete, said anchor comprising: at least one rod member` having a threaded coupling portion and an engaging portion extending from said coupling portion, said engaging portion having undulations of increasing ratio of amplitude to period proceeding from said coupling portion, said undulations being formed with respect to an axis extending in tangential relationship to the axis of said coupling portion.

5. An anchor for engagement with poured concrete,

said anchor comprising: at least one rod member having yan end provided with coupling means and an engaging portion extending from said end, said engaging portion having undulations of decreasing period at constant amplitude proceeding from said coupling portion.

6. An anchor for engagement With cast material, said anchor comprising: at least one rod member having an end provided with coupling means and an engaging portion extending from said end, said engaging portion having undulations ofdecreasing period proceeding from said coupling portion.

7. Anfanchor for engagement With cast material, said anchor comprising: at least one rod member having an end provided with coupling means and an engaging portion extending from said end, said engaging portion having undulations of increasing ratio of amplitude to period proceeding from said coupling portion.

S. An anchor for engagement with east material, said anchor comprising: at least one rod member having an endprovided with coupling means and an engaging portion extending from said end, said engaging portion having undulations of increasing curvature proceeding from said coupling portion.

References Cited in the le of this patent UNITED STATES PATENTS 2,724,165 Williams Nov. 22, 1955 2,788,652 Pilling Apr. 16, 1957 FOREIGN PATENTS 71,212 Austria Feb. 25, 1916 834,613 France a of 1938 

1. AN ANCHOR FOR ENGAGEMENT WITH POURED CONCRETE, SAID ANCHOR COMPRISING: AT LEAST ONE ROD MEMBER HAVING A THREADED COUPLING PORTION AND AN ENGAGING PORTION EXTENDING FROM SAID COUPLING PORTION, SAID ENGAGING PORTION HAVING UNDULATIONS OF INCREASING CURVATURE PROCEEDING FROM SAID COUPLING PORTION, SAID UNDULATIONS BEING FORMED WITH RESPECT TO AN AXIS HAVING CONTINUOUS CURVATURE AND EXTENDING IN TANGENTIAL RELATIONSHIP TO THE AXIS OF SAID COUPLING PORTION. 